LOS ANGELES (Reuters) - When red-hot boy band the Jonas Brothers debut their 3-D movie on Friday in many sold-out U.S. theaters, it will mark a new beginning for them and, perhaps, the end of their youthful innocence.

So far Kevin, 21, Joe, 19 and Nick, 16 have mostly avoided the pitfalls of celebrity -- constant hounding by paparazzi, names trashed in tabloids and on gossip websites, and public scrutiny of personal choices, to name a few.

But the Disney movie "Jonas Brothers: The 3D Concert Experience," which shows them onstage and behind the scenes of their 2008 "Burning Up" tour, could change that. After all, one of their trailblazers, Miley Cyrus (a.k.a. Hannah Montana), saw her world change while on a similar trajectory to stardom.

Right now the trio of pop stars, who gained fame on the Disney Channel's "Hannah Montana" TV show, do not seem worried. In fact, they shrug off what little downside they've seen to stardom -- months away from home on concert tours, long days promoting records and gossip in tabloids and on blogs.

"You just, kind of, have to laugh it off. There's always going to be haters, but that's what comes with the territory of this," said Joe.

"It's weird. You look at a downside, and it depends on your outlook. There was one year where we spent eight months living out of a suitcase, but to us that was exciting," he added.

In a recent interview, the three brothers credited their father, Kevin, with preparing them for the rigors of stardom.

Nick also said his work in Broadway musicals such as "Les Miserables" and "Annie Get Your Gun" offered lessons on the long days and hard work of performing for a living.

TAMING THE MOVIE BEAST

But being a movie star can be a different celebrity beast because more than theater or TV, movies put entertainers on a world stage and expose them to new fans and greater scrutiny.

Miley Cyrus found out. She had a squeaky clean public image from "Hannah Montana," in which she plays a teen living a dual life as a high school student by day and pop star by night. The show also helped give the Jonas Brothers their start in show business.

Last February, Disney released the movie, "Hannah Montana/Miley Cyrus: Best of Both Worlds Concert Tour," to sold-out crowds, raking in $70 million at global box offices.

Her concert tour that proceeded it saw scalpers selling tickets for up to $3,000 with an average price of $240.

But in April 2008 came a Vanity Fair photo that showed the then 15-year-old Cyrus draped in a sheet showing her bare back. Parents complained, and Cyrus was forced to apologize.

About the same time, there were other suggestive, personal photos of her posted online and this year, personal pictures of her making a slant-eyed face that upset Asian-Americans.

No one knows what's ahead for the Jonas Brothers, but the tickets to their movie have been red hot sellers.

Online seller Movietickets.com reported that 700 U.S. theaters were sold out as of mid-week this week, and a Fandango.com survey of 5,000 fans -- 76 percent 17-years-old or younger -- showed 96 percent were planning to see the movie this weekend.

"This year has been a great year for us so far. We're just blown away with the amazing opportunities we are getting," said Kevin, with the upbeat attitude that permeates their music.

"I think it's all happened so fast, although it has been three, four or five years we've been doing this," added Nick. "We're still pinching ourselves and thinking, 'is this really happening?'" (Editing by Jill Serjeant and Vicki Allen)